Laboratory Studies

Culture is the criterion standard method of identification. Cultures are specific but not sensitive and may be positive in only 70-80% of cases.
[19] Cultures at 25-32°C (77-89.6°F) may grow nonmotile acid-fast bacilli in 7-21 days, with an optimal growth temperature of 30°C.
[7] The organisms are photochromogens (Runyon group 1), producing yellow pigment only when exposed to light. No niacin or nitrate production occurs; urease is produced, but the organism is a weak producer of catalase at 25°C (77°F).
[20] It grows in all the media used for mycobacterial growth (egg based, broth, or agar based) without additives or only 2-5% oleic acid-albumin-dextrose-catalase instead of 10% for M tuberculosis, and it also grows on blood-containing agar. In addition, 2-5% carbon dioxide in the gas phase above the medium improves the growth of M marinum.
[7]

If culture results are negative but the history and clinical findings are consistent with M marinum infection, then treatment should still be strongly considered; additionally, performing a lesional biopsy may help identify the organisms with an acid-fast bacillus stain. Of note, tuberculin skin testing is usually positive, owing to cross-reaction with M tuberculosis. This may suggest an underlying mycobacterial infection but cannot distinguish M marinum from tuberculosis or other mycobacterial infections and is of little utility in the workup.
[7]

Polymerase chain reaction (PCR) studies of tissue are quick to perform and are increasingly being used to help distinguish the exact Mycobacterium species involved.
[21] However, errors of identification have been reported when using PCR.
[22, 23]

An enzyme-linked immunospot assay for interferon-gamma has been developed for Mycobacterium tuberculosis. Patients with infection by M marinum may also have a positive result with this test.
[24]

Imaging Studies

MRI may show exuberant tenosynovitis; however, unlike with purulent tenosynovitis, the underlying muscles and bony structures are rarely involved.

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Other Tests

Mycobacteriophages are viruses that infect Mycobacterium. They are currently commercially available but are still being refined as a tool to rapidly identify the specific species in mycobacterial infections. They are also being developed as a future modality for diagnosis and treatment of these infections.
[26]

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Procedures

Biopsy samples from cutaneous lesions or from intra-articular tissue should be obtained for histopathologic analysis. Acid-fast preparations, such as Ziehl-Neelsen and Fite stains, reveal acid-fast bacilli; however, staining is positive in only 30% of the cases. When positive, smear microscopy cannot distinguish M marinum from other mycobacteria.
[7]

Surgical drainage of skin lesions often is unnecessary; however, if a deeper infection is diagnosed, drainage may be indicated.

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Histologic Findings

The histopathologic findings vary depending on the duration of the lesion sampled and on the degree of granuloma formation. Histopathology provides diagnosis of mycobacteriosis in only half of the cases since histologic findings depend on the age of the lesion.
[7]

Granulomas can vary from poorly formed, consisting of epithelioid histiocytes and lymphocytes with few multinucleated giant cells, to fully formed granulomas with numerous multinucleated giant cells, although caseation is rare.
[27] Granulomas are present in less than two thirds of cases and can be confused with rheumatoid nodules.
[28]

A lichenoid and granulomatous infiltrate has also been reported with M marinum infection.
[16]

The epidermis frequently demonstrates papillomatosis, hyperkeratosis, and an acute inflammatory infiltrate, with or without ulceration.

Importantly, one study showed that in one case out of five, the infiltrate suggested no infectious origin, although deep skin biopsies and synovial biopsies provided more information. Therefore, for all forms of necrotic granuloma, whether or not accompanied by collections of neutrophils, a culture should be carried out in a specific medium, even in the absence of microscopic evidence of bacilli.
[28]